As a result of recent advances in electronic technology, it has become possible to produce very compact devices which can perform high-speed data processing and which can moreover handle relatively large amounts of data. Such devices include electronic calculators, certain types of electronic timepiece, etc. However, until now, there has been no compact and easily produced device whereby data can be input as well as output from such types of data processing equipment. Various types of device can provide what is sometimes referred to as "hard copy" output from data processing equipment, e.g. printers and plotters. Temporary data output, i.e. "soft-copy" output can be provided by such devices as CRTs and by electro-optical display devices such as liquid crystal displays. It would be highly desirable, however, to provides some means whereby data could be written into data processing equipment such as an electronic calculator, with the write-in means being of suitably compact dimensions, i.e. of the order of dimensions which can be achieved for read-out purposes by using electro-optical display means. This would enable conversational interaction between the user and the compact data-processing device, in a similar manner to that which is possible with a computer and a CRT monitor provided with light-pen input means. Use of a CRT as a write-in device for a portable electronic device is however impractical, due to the high voltages which are required for the CRT operation, and the high level of power consumption. From the point of view of low power consumption and low levels of operating voltage, liquid crystal display devices are ideal. Liquid crystal displays of matrix type have been developed wherein data can be displayed with a very high degree of display density. Liquid crystal displays also have the advantage of being simple and economical to manufacture on a large scale production basis, and can be made extremely compact. Thus, in almost every respect, they are ideally suited for use in portable devices. In particular, a type of liquid crystal matrix display has been developed in which an individual switching element is provided to control each element of the display. Such a liquid crystal display has a performance which is comparable to that of a CRT, for certain applications.
The problem of inputting data to a portable data processing device however has proved more difficult to overcome. In devices such as calculators or timepieces, data input is generally performed by actuation of a plurality of keys coupled to switches. However, due to the small size of such portable devices, the volume which is available to provide such switches is very limited. In addition, if the keys are of small size, their operation may be difficult and inconvenient. There is therefore a requirement for some means of data input to a portable electronic device which would eliminate the need for key switches, yet which would be simple to use and could be economically combined with the overall system of the portable electronic device. Such a data input means is revealed by the present invention, which enables a liquid crystal display panel to be produced whereby data, including numerals, characters, diagrams etc, can be written into a data processing device by suitable application of a light source (such as a light pen) to the display panel. The display panel can be manufactured in a very similar way to that of a liquid crystal display panel of the prior art, in which a switching element is built-into the display panel for each of the display elements, and the elements whereby the light data input is sensed can be formed by the same manufacturing process, and during the same processing steps, by which the switching elements are formed. A display panel according to the present invention can therefore be produced virtually as economically as can a liquid crystal display panel of the built-in switching element type which does not possess a data write-in function.